The present invention is generally directed to switches for use in superconducting current paths. More particularly, the present invention is directed to a superconducting loop switch which is capable of exhibiting rapid turn-on and turn-off responses so as to be able to permit precise control of the current in the loop.
In applications requiring a highly stable magnetic field, such as in nuclear magnetic resonance (NMR) imaging or spectroscopy, it is customary to join the two ends of a superconducting electromagnet to each other through a length of superconductive material configured to operate as a superconducting switch. Any electrical joints in the circuit are also made to be superconducting so that once a current is established in such a magnet, it can be maintained constant for very long periods of time. The aforementioned switch is referred to herein as a main switch. It has two possible functions, both of which are based on its ability to be switched from a superconducting state to a conventional resistive state. This main switch is in effect connected across the terminals of the superconductive electromagnet. Once a power supply has established a desired current level in the superconducting loop, the main switch is switched from the resistive state to the superconducting state. Once this transition occurs, the current in the superconducting electromagnet coil can no longer change. The current from the power supply may be reduced to zero, establishing a current in the loop comprising the switch and the coil. This current is referred to as a persistent current the properties of which are more particularly described below. Additionally, the main switch may also be switched to the resistive state, for example by heating a part of the superconductive element. The main switch may actually be configured from part of the same superconductive conductor employed in the electromagnet coil or coils. It is desirable that the heater power required to maintain the superconductive switch in the resistive state be moderate, both to keep the switching power supply small and, more importantly to minimize coolant consumption (boil-off). Accordingly, the main switch is disposed so as to be thermally insulated from the cryogenic coolant. The present invention, however, is not directly concerned with the construction of this main switch, but rather with the construction of a second switch to be employed in the superconducting loop so as to be able to permit precise control of the superconducting current.
Although the superconducting phenomenon gives rise to very stable magnetic fields exhibiting deviations of less than one part in ten million per hour, the ability to initially determine the current and field value is limited by the power supply. In particular, the setability of the current and field level is limited by the setability of the power supply. Power supplies, particularly constant current power supplies, are generally limited to an accuracy of no more than one part in 1,000 or one part in 10,000. This is particularly true with respect to power supplies providing the level of superconductive current considered herein. Typically the current levels considered herein for the main electromagnet are between approximately 1,000 amperes and 2,000 amperes. However, in certain applications, such as NMR imaging, it is desirable to be able to set the current more accurately, for example, to one part in one million. Such an accuracy in current setability is not achievable simply through the use of the main switch described above. In particular, since this switch must be thermally isolated from the cryogenic medium, its response time characteristics cannot be sufficiently controlled to effect the desired current adjustment. In particular, it is commonly the case that once a transition to the resistive state has been initiated, a main switch cannot be returned to the superconducting state independently of the heater input, until after the main coil circuit current has been essentially reduced to zero.
Accordingly, it is seen that there is a need for precise adjustment of the current in a superconducting loop. This precision is also seen not to be readily providable through the control of the power supply or the main current switch.